In recent years, speed-enhancement of electronic circuits has been required according to increase in speed and capacity of networks. Circuits used for data conversion, such as an A/D converter circuit and a track/hold circuit (sample/hold circuit), have also been required to operate at higher speeds.
Some circuits and/or configurations have recently been proposed to meet such demands. For example, as one of techniques to achieve high-speed sampling of an A/D converter apparatus, there has been proposed a time-interleaving method of operating a plurality of A/D converter circuits operable at a low sampling rate in a predetermined order and operating the A/D converter circuits in cooperation with each other so that the entire apparatus equivalently achieves high-speed sampling.
FIG. 6 shows an example of an A/D converter apparatus using a time-interleaving method. In this method, a plurality of A/D converter circuits or track/hold circuits are formed in parallel as a plurality of channels (AD converters 1 to 4 in FIG. 6) so as to equivalently provide an A/D converter apparatus having a sampling rate that is the same number of times as the number of the channels (four times in FIG. 6). FIG. 7 shows a timing chart of the A/D converter apparatus. With this method, there can be provided an A/D converter apparatus that can perform a high-speed sampling operation without any limitations on high-speed performance of devices used as parts of the A/D converter apparatus.
Along with recent trends of miniaturization and performance enhancement in semiconductor processes, A/D converter apparatuses that can perform such a high-speed sampling operation have further been enhanced in performance. Additionally, a high-speed signal processing system, which has been implemented only by analog circuits, can be achieved by converting an analog signal into a digital signal with an A/D converter apparatus and then conducting digital signal processing. Thus, there has been developed a signal processing system having performance higher than ever. For example, in a field of ultrahigh-speed optical communication in which a data rate of 40 Gb/s or higher has been required for a high speed and a large capacity, introduction of a digital coherent system that can improve performance and achieve longer distance transmission by using digital signal processing with an A/D converter apparatus has actively been promoted.
A feature of an A/D converter apparatus used in a digital coherent communication system of an optical communication field is that an A/D converter apparatus is required to have a slew rate that is higher than a data transmission speed. This is because data should have redundancy due to an error correction technique or the like even if a multi-valued and high-density modulation method, such as phase shift keying, is used. For example, a slew rate around 30 Gb/s is required even in a case where a data transmission speed is 40 Gb/s (20 Gbps in parallel for 40 G). Furthermore, 2× oversampling performance, which is twice the number of channels, is generally is used as a sampling rate of an A/D converter circuit.
Upon application to an optical communication system, an A/D converter apparatus is required to have such sampling performance for the following reasons: In ultrahigh-speed optical communication, a waveform distortion resulting from a dispersion phenomenon in an optical fiber (e.g., a chromatic dispersion or a polarization mode dispersion) becomes a cause of considerable limitations to a transmission distance. In order to compensate a waveform distortion with a high degree of precision by signal processing, 2× oversampling performance is roughly required in general.
As described above, application of a digital signal processing method along with speed-enhancement of an A/D converter apparatus has been active in an ultrahigh-speed communication field such as optical communication. A time-interleaving method using a plurality of parallel A/D converter circuits has attracted much attention as one of techniques for achieving a high-speed A/D converter apparatus. Meanwhile, there is a problem that error components (variations) such as gains or offsets between a plurality of A/D converter circuits increase noise or distortion and thus deteriorate the conversion accuracy of the entire A/D converter apparatus.
A configuration illustrated in FIG. 8 has been proposed as means for calibrating gain and offset error components between a plurality of A/D converter circuits (Patent Literature 1). The calibration method of this invention includes providing means for generating a sine wave signal as a training signal for calibration (reference signal generator), performing a sine curve fitting process on a series of conversion data with the sine wave signal for each of the A/D conversion means (ADC1, ADC2) to calculate calibration values of gains and offsets, and storing the calibration values in a calibration memory. Upon normal A/D conversion, data are calibrated in accordance with the calibration values stored in the calibration memory.
However, an interleaving method disclosed in Patent Literature 1 originally assumes that signals have a data rate of about several tens of Mbps to about several hundreds of Mbps. For example, if an ultrahigh-speed optical communication of 40 Gbps is subjected to AD conversion with the interleaving method of Patent Literature 1, a data rate should be 40 times or more as high as 40 Gbps. Therefore, the scale of a circuit such as correction means and a calibration memory for maintaining the signal quality becomes so enormous that electric consumption increases considerably.
As shown in FIG. 9, for example, there has been proposed a method of correcting an input signal in the entire time-interleaved A/D converter apparatus (Patent Literature 2). The method of correcting an input signal as disclosed in Patent Literature 2 includes integrally calibrating individual output digital signals of parallel A/D converter circuits (ADC0-ADC3) of the time-interleaved A/D converter apparatus. For this purpose, a high-speed low-resolution correction A/D converter circuit (ADC4) operable at the same rate as an equivalent sampling rate obtained upon time-interleaving and a nonlinear filter are added in addition to the parallel A/D converter circuits (ADC0-ADC3). Upon AD conversion in the time-interleaved A/D converter apparatus, output signals (SIG0-SIG3) of the parallel A/D converter circuits are synthesized, then passed through the nonlinear filter, and corrected by a reference signal (instruction signal: d[n]) generated by the correction A/D converter circuit. With such an operation, the digital signals outputted from the individual parallel A/D converter circuits are corrected at the same timing with the reference signal sampled by the correction A/D converter circuit. In other words, the output signals of all of the parallel A/D converter circuits are sequentially calibrated with the output signal of the correction A/D converter circuit.
However, one of primary factors to use an interleaving method is that a time-interleaving method is used because a single A/D converter circuit cannot achieve a desired high speed. In contrast, implementing a high-speed A/D converter circuit (correction A/D converter circuit: ADC4) operable at the same speed (fs×N) as an equivalent sampling rate obtained upon time-interleaving as disclosed in Patent Literature 2 is not preferable for reducing of the circuit scale by using a time-interleaving method. Furthermore, since a high-speed operation is performed, the electric consumption considerably increases in the correction A/D converter circuit.
As described above, when a communication method of high-precision digital signal processing with A/D converter circuits is applied to current high-speed optical communication systems, the circuit scale and electric consumption problematically increase in the following two aspects:
1) In ultrahigh-speed optical communication, a waveform distortion resulting from a chromatic dispersion, a polarization mode dispersion, or the like in an optical fiber becomes a cause of limitations on a transmission distance. In order to compensate such a waveform distortion with a high degree of precision by signal processing, 2× oversampling performance is roughly required in general. Therefore, increase of the circuit scale and electric consumption is caused.
2) In most cases, high-speed sampling performance of an A/D converter apparatus is achieved by an interleaving method. Meanwhile, correction means for ensuring the signal quality is also provided in many cases. When the correction means is taken into account, the circuit scale and electric consumption considerably increase in the entire A/D converter apparatus.